In recent years, a rapidly increasing number of household electronic devices have become cordless and portable, and as power sources for operating these devices, a growing number of small-sized lightweight non-aqueous electrolyte secondary batteries with a high energy density are in demand. From this perspective, expectations have been placed on non-aqueous electrolyte secondary batteries, especially lithium secondary batteries with a high voltage and high energy density, and the developments thereof have been accelerated.
A battery comprising a lithium-containing composite oxide as a positive electrode active material and a carbon material as a negative electrode active material has recently been attracting attention as a lithium secondary battery with a high energy density. As the lithium-containing composite oxide, LiCoO2 has come into practical use. Although vigorous attempts have been made to put LiNiO2 into practical use with the aim of achieving a higher capacity, LiNiO2 has a problem of having low thermal stability, whereby there would be considerable difficulty in realizing the practical use thereof.
These positive electrode active materials repeat expansion and shrinkage by charging and discharging. At that time, distortion of a crystalline lattice, destruction of a crystal structure or cracking of a particle may occur in the positive electrode active material, resulting in a decrease in discharge capacity.
In order to avoid this, therefore, efforts have been made to attempt to stabilize a crystalline lattice by replacement of part of cobalt with another element so that a cycle life characteristic can be improved.
For example, Japanese Laid-Open Patent Publication No. Sho 63-121258 and Japanese Laid-Open Patent Publication No. 2001-319652 propose a positive electrode active material with part of cobalt replaced with an additional element, which is obtained by mixing a lithium compound, cobalt oxide and a compound of the additional element and then baking the mixture. Such proposals allow a certain degree of improvement in cycle life characteristic. As the additional element employed is an element having the effect of improving the cycle life characteristic, such as Al, and an element having the effect of improving thermal stability of a positive electrode active material, such as Mg.
In the above conventional method, however, the additional element tends to be distributed more in a surface portion of the positive electrode active material since a reaction occurs between solid phases. When the element having the effect of improving thermal stability is distributed more in the surface portion, the effect of improving thermal stability decreases, and thereby desired battery characteristics cannot be obtained. Accordingly considered has been a method comprising preparing a cobalt compound containing an additional element by a coprecipitation method, and baking this cobalt compound and a lithium compound. When a cobalt compound containing Al or the like is prepared by the coprecipitation method, however, the compound has considerably a low tap density. This may result problematically in a low tap density of a positive electrode active material and a small battery capacity.